Alkaline secondary batteries typified by a nickel-hydrogen battery and nonaqueous electrolyte secondary batteries typified by a lithium ion battery are widely used as power supplies for driving portable electronic equipment such as cell phones including smartphones, portable computers, PDAs, and portable music players, alkaline secondary batteries typified by a nickel-hydrogen battery and nonaqueous electrolyte secondary batteries typified by a lithium ion battery are widely used. In addition, alkaline secondary batteries and nonaqueous electrolyte secondary batteries are also widely used for power supplies for driving electric vehicles (EVs) and hybrid electric vehicles (HEVs, PHEVs) and in stationary storage battery systems for suppressing the variation in output power of photovoltaic generation, wind power generation and the like, and for peak shifts in system power in order to store electric power during the night time and to use the electric power during daytime.
A single secondary battery has a low electromotive force and even a lithium ion secondary battery that is considered to have a comparatively high electromotive force has an electromotive force of about 4 V. For using such a battery for vehicles such as EVs, HEVs, and PHEVs that need high capacity and high output characteristics, each battery is upsized, and a number of batteries are connected in series or parallel to form a battery pack as shown in, for example, US 2010/316906 (A1) and US 2008/299453 (A1). To address this, in these applications, prismatic secondary batteries are typically used from the viewpoint of space efficiency.
Examples of such a battery pack include a battery pack in which each battery terminal is extended, the extended terminal is bent, overlapped onto an adjacent battery terminal, and welded or bolted for connection, or include a battery pack in which battery terminals adjacent to each other are connected through a connection member such as a bus bar by welding or bolting. Among them, in a prismatic secondary battery in which a terminal is connected to an adjacent battery terminal through a connection member such as a bus bar, as shown in US 2010/316906 (A1) for example, a battery is known in which each of an external positive electrode terminal and an external negative electrode terminal is formed of a conductive member having a bolt, and the connection member such as a bus bar is bolted directly to the battery.
The prismatic secondary battery having an external positive electrode terminal and an external negative electrode terminal that are formed of the conductive member having a bolt can sufficiently reduce the contact resistance between the bus bar and the external terminal of each battery, thereby achieving a reduction in the internal resistance of a battery pack and high reliability. However, in order to sufficiently reduce the contact resistance between the bus bar and the external terminal of each battery, the bus bar is required to be strongly bolted to the external positive electrode terminal or the external negative electrode terminal.
The external positive electrode terminal and the external negative electrode terminal that are connection points for connecting batteries through the bus bar are electrically connected to an internal positive electrode terminal and an internal negative electrode terminal, respectively, that are relay points of output power from inside of the battery. At that time, in order not to cause short circuit between a sealing plate and the external positive electrode terminal and between a sealing plate and the external negative electrode terminal, each of the external positive electrode terminal and the external negative electrode terminal is disposed on a surface of the sealing plate through an insulating member.
When the bus bar is connected to the external positive electrode terminal or the external negative electrode terminal formed of the conductive member having the bolt in order to form a battery pack, bolting generates rotary torque. Thus, in a conventional prismatic secondary battery, the insulating member has a back face on which a concave portion is provided for suppressing rotation, and the sealing plate has a front face on which a protrusion is provided for suppressing rotation, where the concave portion and the protrusion are opposed to each other. The protrusion for suppressing rotation provided on the front face of the sealing plate is formed by forging concurrently with the formation of a pair of mouths for attaching a positive electrode terminal and a negative electrode terminal, an electrolyte pour hole, and a gas release valve, for example.
The method for forming, for example, the protrusion and the mouths on the front face of the sealing plate by forging enables the formation at relatively low cost but results in a protrusion having an R-shaped end due to the characteristics of forging. Thus, the protrusion formed by forging on the front face of the sealing plate has an R-shaped top end, and such a shape reduces the area of a protrusion face in contact with the concave portion of the insulating member in a longitudinal direction, thereby reducing the rotation suppressing effect of the protrusion. In addition, in conventional prismatic secondary battery, the protrusion of the sealing plate is provided at one position, and the concave portion of the insulating member is provided at one position with respect to each of the external positive, electrode terminal and the external negative electrode terminal. Hence, when the bus bar is bolted to the external positive electrode terminal or the external negative electrode terminal, a load is concentrated to the insulating member thereby to deform the insulating member.